entry: Add arch_in_rcu_eqs()

All architectures have an interruptible RCU extended quiescent state
(EQS) as part of their idle sequences, where interrupts can occur
without RCU watching. Entry code must account for this and wake RCU as
necessary; the common entry code deals with this in irqentry_enter() by
treating any interrupt from an idle thread as potentially having
occurred within an EQS and waking RCU for the duration of the interrupt
via rcu_irq_enter() .. rcu_irq_exit().

Some architectures may have other interruptible EQSs which require
similar treatment. For example, on s390 it is necessary to enable
interrupts around guest entry in the middle of a period where core KVM
code has entered an EQS.

So that architectures can wake RCU in these cases, this patch adds a
new arch_in_rcu_eqs() hook to the common entry code which is checked in
addition to the existing is_idle_thread() check, with RCU woken if
either returns true. A default implementation is provided which always
returns false, which suffices for most architectures.

As no architectures currently implement arch_in_rcu_eqs(), there should
be no functional change as a result of this patch alone. A subsequent
patch will add an s390 implementation to fix a latent bug with missing
RCU wakeups.

[[e-mail]: rebase, fix commit message]

Signed-off-by: Mark Rutland <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Christian Borntraeger <[e-mail]>
Cc: Heiko Carstens <[e-mail]>
Cc: Paolo Bonzini <[e-mail]>
Cc: Paul E. McKenney <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Sven Schnelle <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Claudio Imbrenda <[e-mail]>
Cc: Vasily Gorbik <[e-mail]>
Cc: Alexander Gordeev <[e-mail]>
Cc: Janosch Frank <[e-mail]>
Reviewed-by: Christian Borntraeger <[e-mail]>
Signed-off-by: Andrew Donnellan <[e-mail]>
Acked-by: Peter Zijlstra (Intel) <[e-mail]>
Reviewed-by: Janosch Frank <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Janosch Frank <[e-mail]>
Message-ID: <[e-mail]>

include/linux/entry-common.h

···
 #endif
 
 /**
+ * arch_in_rcu_eqs - Architecture specific check for RCU extended quiescent
+ * states.
+ *
+ * Returns: true if the CPU is potentially in an RCU EQS, false otherwise.
+ *
+ * Architectures only need to define this if threads other than the idle thread
+ * may have an interruptible EQS. This does not need to handle idle threads. It
+ * is safe to over-estimate at the cost of redundant RCU management work.
+ *
+ * Invoked from irqentry_enter()
+ */
+#ifndef arch_in_rcu_eqs
+static __always_inline bool arch_in_rcu_eqs(void) { return false; }
+#endif
+
+/**
  * enter_from_user_mode - Establish state when coming from user mode
  *
  * Syscall/interrupt entry disables interrupts, but user mode is traced as

kernel/entry/common.c

···
 	 * TINY_RCU does not support EQS, so let the compiler eliminate
 	 * this part when enabled.
 	 */
-	if (!IS_ENABLED(CONFIG_TINY_RCU) && is_idle_task(current)) {
+	if (!IS_ENABLED(CONFIG_TINY_RCU) &&
+	    (is_idle_task(current) || arch_in_rcu_eqs())) {
 		/*
 		 * If RCU is not watching then the same careful
 		 * sequence vs. lockdep and tracing is required